In recent years, with the widespread use of cordless equipment such as personal computers and mobile phones, there has been an increasing demand that batteries serving as their power sources have a smaller size and a higher capacity. Owing to its high energy density and its potential for reduction in size and weight, a lithium ion secondary battery is now put to practical use and growing in demand as a portable power source. However, this lithium ion secondary battery cannot ensure sufficient hours of continuous use for some types of cordless equipment.
In an attempt to solve the problem mentioned above, a fuel cell, for example, a polymer electrolyte fuel cell (PEFC) has been under development. The fuel cell can be used continuously as long as a fuel and oxygen are supplied. The PEFC, which uses a solid polymer electrolyte as an electrolyte, oxygen in the air as a positive active material and various kinds of fuels as a negative active material, has attracted attention as a battery achieving a higher energy density than the lithium ion secondary battery.
As the fuel to be used in the PEFC, hydrogen, methanol, etc. have been proposed and developed. Among them, a PEFC using hydrogen as its fuel offers promise because it enables a higher energy density.
In order to supply the fuel cell such as the PEFC with hydrogen, studies have been conducted on, for example, a method of supplying the hydrogen that is produced using a hydrogen producing apparatus capable of allowing a hydrogen generating material serving as a hydrogen source and water to react with each other so as to generate hydrogen. For instance, such a hydrogen producing apparatus is provided with a tank containing the water and a vessel containing the hydrogen generating material and allowing the hydrogen generating material and water to react with each other and has a mechanism in which the water is supplied from the tank containing the water to the vessel containing the hydrogen generating material (a reactor), the hydrogen generating material and the water are caused to react with each other in that vessel, and the generated hydrogen is supplied to the fuel cell via a hydrogen outflow pipe provided in this vessel.
However, at the time of reaction between the hydrogen generating material and the water, unreacted water is ejected together with the hydrogen and discharged to the outside of the reactor. Accordingly, in the conventional hydrogen producing apparatus, the water containing tank has had to hold water in an amount considerably larger than the amount necessary for reaction. Since hydrogen containing such a large amount of water is supplied, a fuel supply line in the fuel cell sometimes has been clogged.
Under such circumstances, in the hydrogen producing apparatus having a mechanism of generating hydrogen by reaction between the hydrogen generating material and water, the unreacted water discharged from the reactor together with the generated hydrogen has to be treated. In response to this, suggestions conventionally have been made to provide a hydrogen producing apparatus having a mechanism in which a mixture of the hydrogen and water (water vapor) discharged from the reactor is returned to the inside of the water containing tank and mixed with the water inside the tank so as to conduct heat exchange, thereby converting the water vapor in the mixture into water, whereas the hydrogen in the mixture is taken out from an outflow pipe provided in an upper portion of the tank and supplied to a fuel cell, etc., for example (Patent document 1). In accordance with the apparatus described in Patent document 1, it is possible to convert the unreacted water discharged from the reactor as water vapor together with the hydrogen back to water inside the water supply tank and supply the water to the reactor again, so that the water can be used efficiently for producing the hydrogen.
However, the water supply tank in Patent document 1 needs to include a pipe for supplying water, a pipe for returning the mixture of hydrogen and water discharged from the reactor and a pipe for discharging hydrogen, so that the structure of the water supply tank becomes complicated. Since the hot mixture of hydrogen and water is returned directly to the water supply tank, the temperature of water inside the water supply tank rises along with the hydrogen production. Accordingly, the water supply tank has to be made of a heat resistant material. Thus, production of the water supply tank with a simple structure using an inexpensive material becomes difficult.
On the other hand, methods conventionally have been suggested in which hydrogen is generated by chemical reaction at a low temperature up to 120° C. and used as a fuel. These methods employ as a hydrogen source a metal that reacts with water and generates hydrogen, for example, aluminum, magnesium, silicon or zinc (Patent documents 2 to 6).
However, in the case of producing hydrogen by the methods disclosed in Patent documents 1 to 6, by-products such as oxides and hydroxides are generated by the above-noted reaction between metals and water. In the produced hydrogen, unreacted water that has not acted in the hydrogen generating reaction is mixed in such a manner as to contain the by-products mentioned above and ions thereof.
When the water containing the above-mentioned by-products and ions thereof is supplied to the polymer electrolyte fuel cell together with hydrogen, protons in a proton exchange resin and a proton exchange membrane included in the polymer electrolyte fuel cell are substituted, adsorbed on a catalyst and deposited in electrodes. This lowers ion conductivity, catalyst function, gas diffusion performance, etc. in the fuel cell, so that the fuel cell is deteriorated.
In light of the above situation, in the case where hydrogen serving as a fuel of the polymer electrolyte fuel cell is obtained by the methods disclosed in Patent documents 1 to 6, water containing the by-products mixed in the hydrogen and the ions thereof is preferably removed in order to prevent the properties of the fuel cell from deteriorating.
Patent document 5 mentioned above has suggested a method in which the by-products contained in the generated hydrogen and the ions thereof are separated together with condensed water using a condensed liquid separating device. Most of the by-products and the ions thereof contained in the hydrogen are separated with this method.
However, the above-described method only can separate a condensed portion in the water contained in the hydrogen and cannot separate water vapor. Thus, there is a problem that, even if the by-products and ions thereof contained in the condensed water droplets can be removed, part of the by-products and ions thereof contained in the water vapor are sent to the fuel cell together with hydrogen.
Also, Patent document 6 mentioned above discloses a mechanism of removing metal ions contained in the hydrogen using a hydrogen separating membrane such as a palladium membrane, a metal ion permselective membrane, molecular sieves or the like. However, the hydrogen separating membrane such as a palladium membrane has a problem of a low hydrogen permeability rate and a high cost. Further, the metal ion permselective membrane and the molecular sieves have a problem of early deterioration because the amount of metal ions that can be removed is limited.
Patent document 1: JP 2004-149394 A
Patent document 2: U.S. Pat. No. 6,506,360
Patent document 3: JP 2566248 B
Patent document 4: JP 2004-231466 A
Patent document 5: WO 04/18352A
Patent document 6: JP 2002-80202 A